The invention relates to apparatus and methods for liquid gauging, and more particularly to apparatus and methods for remotely interrogating liquid quantity sensors.
Many types of liquid quantity and level sensors are known, including capacitive sensors, resistive sensors, acoustic sensors and so forth. Passive sensors generally operate on the basis of a sensor element that exhibits a parameter, e.g. capacitance, that varies with the liquid level. Active sensors such as acoustic sensors operate on the basis of producing a signal, e.g. an acoustic pulse, that can be used to detect the liquid level by parametric analysis such as echo ranging.
Such systems further include an electronic circuit that detects the parametric value of interest and converts that value to a signal that corresponds to the liquid level.
A common application for such liquid level sensors is for fuel level and quantity detection in aircraft fuel tanks. However, due to the volatile nature of fuel, it is desirable to minimize the connection of electrical energy to the sensors which may be disposed in the fuel. It is further desirable to minimize the amount of electrical energy stored in the sensors or used by the sensors.
A known approach for minimizing the coupling of electrical energy into a fuel tank is described in U.S. Pat. No. 4,963,729, issued to Spillman et al., and commonly owned by the assignee of the present invention. In this system, optical energy is coupled to the sensors via optic fibers. This optical energy is then converted to electrical energy for energizing the sensors. The sensors detect the liquid level and then transmit another optical signal back to a detector via the optic fibers. The detector then converts the second optical signal into an output that corresponds to liquid level in the tank.
For aircraft applications, on board readings often need to be verified by ground crews, either during routine turn around or to confirm an error reading. The optical fiber link to the internal sensor in the above system prevents as a practical matter interrogation of the sensor by ground crews, other than via the same optic fiber link which may in fact be the cause of a fault reading.
A commonly used fuel level sensor in commercial aircraft particularly is a dripstick sensor, which is used as a backup fuel gauging apparatus to the on-board electronic fuel level sensors. For example, dripstick verification may be needed when a refueling truck gauge disagrees with the aircraft fuel gauge, if the on-board fuel gauges appear to be inaccurate or inoperative, or simply by request of the flight crew, among other possible reasons.
The dripstick includes a linear body that extends vertically into the fuel tanks. Often there is a plurality of such dripsticks in each wing of the aircraft. A magnetic float is disposed on the dripstick body like a collar that floats at the fuel surface. The dripstick is read by the ground crew by manually withdrawing the dripstick from the wing until a magnetic tip at the upper end of the sensor body engages the float. The operator can feel the resistance of the tip against the magnetic float and stop pulling on the dripstick. The dripstick body includes a series of markings which visually indicate to the operator the fuel level based on how far the dripstick was withdrawn from the tank. Although dripstick designs may vary somewhat, the basic operation of manual access and visual interrogation is the same for the ground crew.
Various problems are associated with using the conventional dripsticks, especially the time involved for the ground crew to walk around to all the sensors and manually/visually determine the readings. The mechanic climbs a ladder or uses a lifting device to gain close access to the underside of the wing, withdraws the dripstick until the engagement is detected, records the reading and then replaces the dripstick into the tank. Dripstick design is further complicated by the need for minimal fuel leakage. This entire process must be repeated for each sensor, which adds substantially to the refueling operation and turnaround time for aircraft flight readiness.
Commercial air carriers have long identified the need for a dripstick-like backup system, but one that is easier to use. A system that can be interrogated from the ground would eliminate the need for lifting equipment and provide easier reading of difficult access dripsticks. Overall reduction in refueling and fuel verification delays could then be realized.
The need exists therefore for simple and reliable apparatus and methods to interrogate liquid gauging sensors from a remote, preferably ground level, location without coupling electrical energy into a volatile container. Such an arrangement should also be compatible with current dripstick sensor configurations if desired for a particular application.